Water Found in Planet-Forming Disc

by Paul Gilster on October 24, 2011

An orange dwarf star a bit smaller than our Sun is giving us valuable clues about how water-covered planets like Earth may evolve. TW Hydrae is 176 light years away, so young (5 to 10 million years) that it is still in the early stages of forming a planetary system. Working with data from ESA’s Herschel space observatory, astronomers have found cold water vapor in the disc of dust and gas that surrounds the star. It’s a significant find, because while we’ve found warmer water vapor in proto-planetary discs closer to their star, we now see evidence for much larger amounts of water in the outer disc, where the material for icy comets is found. Current theory holds that water will be far scarcer in the inner solar nebula around a coalescing system, meaning extensive oceans would have to be delivered by impacting objects from the outer regions.

The Herschel data show the distinct signature of water vapor, probably produced when ultraviolet radiation from the central star warms ice-coated dust grains, causing some water molecules to break free of the ice to create the thin layer of gas found by Herschel’s Heterodyne Instrument for the Far-Infrared (HIFI). TW Hydrae’s disc extends to 196 AU, and the assumption is that as matter within the disc grows into planets, much of the outer dust and ice will coalesce to become comets. Cometary bombardment in the emerging solar system could then produce oceans on the inner worlds there, a process that this work indicates may be common. Says Caltech’s Geoff Blake, one of the team of researchers investigating TW Hydrae: “These results beautifully confirm the notion that the critical reservoir of ice in forming planetary systems lies well outside the formation zone of Earthlike planets.”

Image: This image shows an artist’s impression of the icy protoplanetary disc around the young star TW Hydrae (upper panel) and the spectrum of the disc as obtained using the HIFI spectrometer on ESA’s Herschel Space Observatory (lower panel). The graph in the lower panel shows the spectral signature of water vapour in the disc. Water molecules come in two “spin” forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. By comparing the relative amounts of ortho and para water, astronomers can determine the temperatures under which the water formed. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. The ratio of ortho to para water observed in TW Hydrae’s protoplanetary disc is low enough to point to the presence of cold water vapour. Credit: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory).

“The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System’s evolution, where over millions of years, similar dust grains then coalesced to form comets. These comets we believe became a contributing source of water for the planets.”

Simulations that folded the new Herschel data in with Spitzer observations as well as ground-based studies allowed the team to calculate the total amount of water in the TW Hydrae disc, an amount equal to several thousand Earth oceans. Studying such raw materials of planetary formation should help us understand how systems evolve, which is why upcoming Herschel studies of three more young stars with similar discs should be so interesting. The expectation is that more water vapor should turn up, supplying additional evidence for the kind of icy reservoir from which water-covered worlds can draw as infant solar systems emerge.

Thomas Gold (The Deep Hot Biosphere) believes hydrocarbons in the Earth’s crust were not formed by primarily by decay of biological matter but have been there since the Earth’s formation and are thus vastly larger than current estimates, seeping up and eventually re-filling drained reservoirs. Is there any evidence of vast amounts of hydrocarbons around newly forming systems?

When speaking of this suspected cometary source of water for a (terrestrial) planet, is this also true for the large amount of water in its mantle, or is that water ‘original’ for the planet (i.e. not coming from comets, but present when the planet was formed from the proplyd)?

simple physics and simple chemistry:
Stars go supernova and dump gigantic amounts of oxygen and carbon into the Interstellar medium, -which condenses into very visible molecular clouds. These must contain hydrocarbons and water because these two elements ( C and O) form very stable compounds with hydrogen. QED wet planets and hydrocarbons frozen into icy bodies, as well as accreted into planets. The scorched body of earths moon has been an unconscious model for the whole universe ( some astronomers acted surprised to find water on mars! ) but in fact it is the moon that is atypical! we only have observed frozen volatiles at the lunar poles — even Mercury has those – We need to get out there and see the universe as it is. Life may not be common but its building blocks sure are.

Evidence like this is extremely encouraging, not just to those hoping life and its building blocks are commonplace, but also to those of us simply interested in the resources we humans need to survive out there being widely available.

Jkittle, yes C and O do form very stable molecules with H, and yes they are very abundant, but, at the high temperatures where new chemicals first form in a protoplanetary disc, it is CO that is virtually the only stable molecule. As temperatures lower there is too little time before the hydrogen disperses to react this carbon monoxide into the significant amounts of water and hydrocarbons that the new equilibrium temperature would suggest unless we invoke complex catalysis schemes. These are difficult to calculate so there is definitely no QED here.

Ronald, it is my understanding that much of that original mantle water is crushed from hydroxides, and I’m wondering if those minerals would be apparent in the above survey.

With regard to Gold’s non-organic origin of hydrocarbons hypothesis: this seems very unlikely to me for coal and oil, since these typically occur in certain geological strata that are all from life-bearing periods, consist of rather long complex molecules and in many cases (particularly in coal) the organic origin can be clearly seen.
In the case of natural gas, which is simply methane (CH4), I could imagine a (partly) abiotic origin, CH4 is very common in gas giants and ice subgiants, be it under much colder conditions.

Fascinating discovery! Makes me wonder about what this system might look like far in the future long after we are gone (though maybe–or at least hopefully–our descendents will be out there among the stars). Perhaps there will be water-rich earth-sized planet in the habitable zone that gives rise to life. Perhaps that hypothetical life will evolve to become intelligent life not knowing its primitive beginnings were be watched, by us.

One of the things I was wondering about as to the issue of finding water in protoplanetary disks is as follows: why has it been easier to find water in the warmer inner regions of these disks where presumably it is less abundant? Another question I have is: is there any indication as to how common are water-rich disks like TW Hydae? An answer to the latter question would give us a strong indication as to how common water worlds may be.

Jkittle, typical reaction schemes in giant molecular clouds and proplyds are wondrous strange. They take some getting use to and are often very different than what we might expect if we mix familiar Earthly chemistry with commonsense. Also the original molecules in a molecular cloud contribute surprisingly little to the later proplyd chemistry.

Ronald, I agree with your objections to Gold’s scheme, yet it explains a couple of deep mysteries so well that I think it deserves a closer look. Namely why are very high levels of helium so often found in natural gas, and why coal seems preternaturally enriched in some un-biological elements – especially measurable are its radionuclides.

“With regard to Gold’s non-organic origin of hydrocarbons hypothesis: this seems very unlikely to me for coal and oil, since these typically occur in certain geological strata that are all from life-bearing periods, consist of rather long complex…”

Ronald, yes that’s the standard theory however the Deep Hot Biosphere model also says that the majority of the Earth’s biomass is in the crust with surface life almost an afterthought, the oil feeds the life below and is not the product of life and that the oil seeps up so it would get into all the upper strata where surface life decayed. Also, I don’t know if coal is part of that model and I suspect conventional processes could exist side by side with Gold’s concept.

Coal is definitely fossil fuel, Cambrian fossils have been found in coal.

Some oil has been found below the basement rocks by the Russians. Read some of Gold’s online stuff to see why he believed abiotic oil was real. Biotic oil isn’t excluded, oil could be dual-sourced.

Most of our analysis depends on either emission lines or absorption lines, it’s easier to find hot or warm molecules because they emit, cold molecules if they’re back-lit by a hot source. Hence it’s easier to figure the composition of the hot parts of a proto-planetary disk.

Larry D you say “Coal is definitely fossil fuel, Cambrian fossils have been found in coal” but Gold points out that some of those fossils (such as trees) are preserved in great detail for several vertical metres. It disturbs him (and has puzzled me since I first heard of this phenomena as a kid) that a deposit could be rendered down to pure carbon content with so little deformation. Gold felt that the answer was that their original peat bogs formed at locations where mashes overlaid upwellings of natural gas. Whatever the answer I would love to see the problem of these unmolested fossils addressed.

Forgive me for not giving a direct link to the paper, but Paul captured the mystery of it all so much better. Anyhow, note that biological activity could only convert methane to hydrogen and carbon monoxide against the chemical potential, and so if this was the agent of that surprise finding, and the exoplanet had similar levels of photosynthetic activity to Earth and devoted half the dark reaction energy to this as an unusual consequence of its existence, then this conversion would take tens of millions of years.

Note how thinking this way, even for speculation, holds the implicit assumption that even at those very high temperatures (1000K) and pressures, carbon monoxide would not back-react with hydrogen to give methane over timescales of a few million years or so. You may feel that this is impossible, but no, even here the conversion will require the presence of suitable catalytic surfaces!

The real problem then in not that GJ 436b is impossible, but that if such surfaces are so rare it the atmospheres of such planets, how do Sol’s giant planets manage to produce so much methane. Paul concluded that article by saying that GL 436b’s atmospheric composition might hint at life there, but I couldn’t help noticing that a better fit was that it hinted of life on Jupiter Saturn Neptune and Uranus (ie, the answer might be that it is GJ 436b may be lifeless).

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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